Blood circulates from the heart through arteries, capillaries and veins and back to the heart to be recirculated. When leaving the heart it measures as a higher (systolic) pressure when the heart contracts, for example in a normal range of about 120 mm Hg, and a lower (diastolic) of about for example in a normal range of 80 mm Hg as the heart relaxes. Measurement of pressure is important as high blood pressure can indicate such problems as kidney disease or toxemia and low blood pressure can indicate shock.
Measurement is usually made with a sphygmomanometer, a device having an inflatable cuff connected to a measuring device, often a clear tube containing mercury. The cuff is placed around a limb and inflated until it compresses an artery until blood flow stops. This temporarily shuts off the blood flow of the artery, and mercury rises in the tube. As the cuff is slowly deflated, mercury drops in the tube. With a stethoscope, the operator listens for the flow of blood to begin, indicating pressure in the cuff is just below the pressure in the artery. At this point, a note of mercury height is made. The cuff is further slowly deflated until the beating sound disappears and blood flows more steadily. This gives a systolic reading. Such a device is uncomfortable and limits readings to highest and lowest, rather than continuous pressures, and is time consuming. Further, locations at which a cuff device can be used are limited to areas where the cuff can be wrapped around a limb. Further the cuff is sensitive to physiology of the subject, requiring different sizes for average, small and large limbs and varies with position of the limb relative to the subject's body.
The diastolic and systolic blood pressures represent approximate points of a continually varying pressure curve.
The two methods of obtaining accurate continuous total blood pressure are surgical insertion of a sensor into the artery or employment of doppler (sound) to measure a series of echoes as blood passes a particular point.
Surgical process is invasive and carries with it all of surgery's undesirable risks including anaesthesia and infection. Doppler devices are considered to be of equal accuracy to pressure sensors inserted surgically, but are limited in requiring large support equipment to receive, analyze, record and report the data obtained. Surgically inserted sensors also have a shelf-life of only a few months and are intended to be non-reusable. Doppler devices require application of conductive ointments to achieve maximum function.
In heart tissue, electrical Charges are continually generated within specific clusters of cells, accumulating an electrical potential in each cluster similar to electrical build-up in a Van de Graaf generator. When the stored electrical charge exceeds the storage capacity of each cluster, the clusters (ideally) discharge simultaneously, causing the heart muscle to contract. As the heart contracts, accumulated fluid (i.e. blood in the heart chamber) is forced through a flexible tube (i.e. artery) and through the pipeline of arteries and capillaries and eventually returns to the chamber through the venous system.
This is mechanically analogous to a piston type pumping system which circulates water in a municipal water supply, as distinguished from a rotating circulating pump.
An electrocardiograph measures the buildup and discharge of electrical charges in the heart, but does not measure the blood pressure.
With each contraction, the heart muscle generates pressure on the blood which is forced into the closed environment of the arterial system which is contained in other tissue resulting in pressure of fluid against the walls of the arterial tubes, creating a contained force at each position along the arterial tubes.
Piezoelectricity is a reversible relationship between mechanical and electrostatic stress exhibited by certain crystals which lack a center of symmetry. For example, when pressure is applied to a piezoelectric crystal such as quartz, positive and negative electric charges appear on opposite crystal faces. Replacing pressure with tension reverses the sign of the electrical charges. Piezoelectric systems are used as sensors because they are sensitive to slight changes in pressure and have an electrical output which is easily amplified for display.
Blood pressure is essentially a varying pressure in a flexible tube encased in a flexible environment where a fluid is moved by varying pressures through the tube. Since piezoelectric crystals respond to mechanical stress, it has been attempted to measure blood pressure noninvasively with piezoelectric crystals. U.S. Pat. No. 4,269,193 discloses such a device. However, gas or air is used to press on the artery, and the transducer must be capable of sensing force applied to the artery. The array uses individual silicon rectangular chips which are relatively difficult and expensive to make and use. Further, the device has proved less than satisfactory.
U.S. Pat. No. 5,033,471 discloses a means for measuring blood pressure, without use of a separate cuff, and suggests possible use of a piezoelectric sensor for detecting a pulse wave. However, a pump for applying pressure is still required.
Other materials than crystals are known which demonstrate piezoelectric properties, or can be manipulated to demonstrate such properties. Such materials are used in sonic sensors. U.S. Pat. No. 4,578,613 discloses an electroacoustic device with two sheets of foil stretched about a curve in perpendicular directions. The foil has been permanently altered to provide directional piezoelectric action on a curved surface. The second foil measures perpendicular strain. A suggested use is amplification of acoustic signals. The piezoelectric material is overstretched precharged polymeric film.
U.S. Pat. No. 4,737,676 discloses a piezoelectric transducer for measuring mechanical quantities in hollow bodies which can be used at temperatures exceeding 80 degrees C. This is given as a temperature which limits piezo-effects in many piezo-polymer materials. Material supporting the metallic portion of the film is flexible and forms a transducer.
U.S. Pat. No. 4,833,659, incorporated herein by reference, shows a sonar system with solid material between piezoelectric films, for damping the signal.
U.S. Pat. Nos. 4,782,469 and 5,159,228 disclose piezoelectric sensors for use in ultrasonic detection designed to withstand shock waves.
There is a need for a suitable piezoelectric sensor and apparatus which is compact, reusable, can externally measure dynamic pressure in a flexible conduit in a flexible environment such as an artery, is usable in a variety of sites, and in human and veterinary practice will directly and continuously monitor arterial pressure and is economical to manufacture.
There also exists a need for a compact apparatus having at least the same measurement capabilities as the inserted sensor and the doppler devices, which will measure pressure force at each point along an arterial tube system and process the measurements into quantifiable data.
The state of the art establishing the need for and lack of blood pressure measurement devices with such capabilities is demonstrated in "DEFENSE TECHNOLOGY CONVERSION, REINVESTMENT, AND TRANSITION ASSISTANCE", Small business innovation Research (SBIR) Program solicitation dated May 1993, at page 19 from the Advanced Research Projects Agency (ARPA) of the Department of Defense and other agencies:"Objective: Advance sensor technology and information technology to speed care to trauma cases. Special emphasis on (1) non-invasive sensing, (2) portable laboratory testing, and (3) medical imaging devices.
Description: There is a need for timely medical information to support decisions at the injury scene/battlefield to plan and manage overall responses. Survival rates decrease dramatically if treatment is delayed for more than one hour . . . . There is a need for non-invasive sensing of vital signs and body chemistry which can acquire information continuously, even prior to injury or illness, and transmit this information . . . ; and advanced, mobile, low powered medical imaging devices that provide for field/remote use."